Measuring mass flow of a fluid, particularly fluids of unknown or varying density, is a difficult task. The difficulty can be compounded greatly when the fluid is a liquid with entrained gas bubbles. When using constant displacement flow meters it is necessary to know or constantly monitor fluid density, and liquid gaseous mixtures are essentially incapable of being accurately measured. In order to solve some of the varying density problems associated with volume measuring devices, coriolis effect meters gained popularity in the early 1980's. While coriolis meters enabled mass flow of fluid in a conduit to be measured directly, coriolis meters have a number of problems, not the least of which are cost, accuracy in the low flow range, and pressure loss in the high flow range. Typical coriolis effect flow meters are illustrated in U.S. Pat. No. 4,422,338 and RE 31,450 of James Smith.
There have also been several attempts to measure mass flow in a closed conduit utilizing momentum techniques. U.S. Pat. No. 3,049,919--Roth and U.S. Pat. No. 3,584,508--Shiba disclose flow meters utilizing a movable U-tube which is provided with a pair of bellows joints to enable the U-tube to deflect, or to alternatively exert a load on the transducer proportional to the momentum of the fluid within the conduit. By knowing or calculating density, mass flow may be determined. Roth utilizes a transducer to weigh the cantileveredly supported U-shaped tube and a liquid contained therein to automatically determine density.
One of the major problems associated with U-tube momentum flow meters utilizing bellows or other expandable joints is inaccuracy resulting from pressure variations of the fluid being measured. Pressure variations cause the bellows to expand or contract. In addition, the shape of a bellows inherently causes problems since solid debris suspended in a fluid may accumulate in the bellows. Problems associated with bellows are serious and greatly reduce the number of applications for flow meters in which bellows are required.